Resource scarcity drives lethal aggression amongprehistoric hunter-gatherers in central CaliforniaMark W. Allena,1, Robert Lawrence Bettingerb, Brian F. Coddingc, Terry L. Jonesd, and Al W. Schwitallae

aDepartment of Geography and Anthropology, California State Polytechnic University, Pomona, CA 91768; bDepartment of Anthropology, University ofCalifornia, Davis, CA 95616; cDepartment of Anthropology, University of Utah, Salt Lake City, UT 84112; dDepartment of Social Sciences, CaliforniaPolytechnic State University, San Luis Obispo, CA 93401; and eMillennia Archaeological Consulting, Sacramento, CA 95817

Edited by Robert L. Kelly, University of Wyoming, Laramie, WY, and accepted by Editorial Board Member Richard G. Klein September 6, 2016 (received forreview May 19, 2016)

The origin of human violence and warfare is controversial, and somescholars contend that intergroup conflict was rare until the emer-gence of sedentary foraging and complex sociopolitical organization,whereas others assert that violence was common and of considerableantiquity among small-scale societies. Here we consider two alterna-tive explanations for the evolution of human violence: (i) individualsresort to violence when benefits outweigh potential costs, which islikely in resource poor environments, or (ii) participation in violenceincreases when there is coercion from leaders in complex societiesleading to group level benefits. To test these hypotheses, we eval-uate the relative importance of resource scarcity vs. sociopoliticalcomplexity by evaluating spatial variation in three macro datasetsfrom central California: (i) an extensive bioarchaeological recorddating from 1,530 to 230 cal BP recording rates of blunt and sharpforce skeletal trauma on thousands of burials, (ii) quantitativescores of sociopolitical complexity recorded ethnographically,and (iii) mean net primary productivity (NPP) from a remotelysensed global dataset. Results reveal that sharp force trauma,the most common form of violence in the record, is better predictedby resource scarcity than relative sociopolitical complexity. Bluntforce cranial trauma shows no correlation with NPP or politicalcomplexity and may reflect a different form of close contact vio-lence. This study provides no support for the position that violenceoriginated with the development of more complex hunter-gath-erer adaptations in the fairly recent past. Instead, findings showthat individuals are prone to violence in times and places ofresource scarcity.

warfare | prehistoric violence | North America

Debate over the antiquity of and explanation for human violenceand warfare is longstanding and highly controversial. Two basic

alternatives have historically dominated: the Hobbesian notion thatcivilization rescued humanity from a long history of “war of allagainst all,” and the Jean-Jacques Rousseau counter that oppres-sion, conflict, and violence were actually caused by civilization andthat less complex societies were marked by greater levels of peaceand harmony (1). Notwithstanding recent anthropological studiessuggesting warfare to be extremely rare among mobile hunter-gatherers (2–7), there is undeniable ethnographic and archaeo-logical evidence for a long history of intergroup violence amongmobile forager societies (8–13), as exemplified by remains froman apparent massacre of mobile foragers in Turkana during theearly Holocene and the somewhat earlier Jebel Sahaba site inJordan (11). Granting that violence and warfare were presentamong ancient small-scale societies (14), the reasons why remainhighly debated. Current explanations for violence among hunter-gatherers focus on two hypotheses that emphasize the causalroles of either resource scarcity or political complexity.The first hypothesis focuses on environmental variables,

building on longstanding anthropological arguments about resourcescarcity and competition (15, 16), but adding the central evolutionarytenant that violence should result from individual self-interest (17–22). Given the obvious costs of engaging in aggression, including the

risk of immediate mortality and long-term reprisals, individualsshould only take up violence when the benefits (e.g., material goods,status, and long-term alliances) outweigh those costs (18–22). Thebenefits are more likely to outweigh the costs when and where en-vironmental productivity is low, resources are scarce, and individualshave relatively more to lose from theft (23). If individual evaluationof the costs and benefits of lethal aggression determines the incidenceof violence, and if these evaluations vary ecologically, then (P1) wepredict that rates of lethal aggression should covary negatively withenvironmental productivity, increasing as productivity decreases.The second hypothesis is sociopolitical and focuses on the

group benefits of violence: even when the potential benefits oflethal aggression do not outweigh its physical cost (of injury ordeath), individuals may nevertheless risk their lives and joinother unrelated individuals in violent conflict that benefits theirsociopolitical group, if members who refuse to fight suffer sig-nificant costs of social punishment (24). If sufficiently severe,community imposed sanctions that enforce participation in lethalaggression, e.g., the ostracizing of cowards (24), may encouragecooperative participation in violence at levels giving these groupsadvantages over groups less able to punish, thus less capable ofviolence (25, 26). This hypothesis implies that violence should bemore common among groups with greater sociopolitical complex-ity, with leaders able to enforce participation through sanctionedpunishment. This line of thinking can be linked to other long-standing anthropological hypotheses about the origins of warfarethat propose that social power differentials allow high-status indi-viduals and leaders to coerce low-status individuals to risk their livesto provide benefits accrued by the high power elite (27–30). If in-dividuals are more likely to engage in lethal aggression under the

Significance

From warfare to homicide, lethal violence is an all too commonaspect of the human experience, yet we still do not have a clearexplanation of why individuals kill one another. We suggestthe search for an answer should begin with an empirical un-derstanding of where and when individuals are more prone toexperience violence. Examining patterns of lethal trauma amonghunter-gatherer populations in prehistoric central California, thisstudy reveals that violence is explained by resource scarcity andnot political organization. This finding provides a clear rationaleto understand why violence may be greater in specific times orplaces through human history, which can help predict where andwhen it may arise in the future.

Author contributions: M.W.A., B.F.C., and T.L.J. designed research; M.W.A., R.L.B., B.F.C.,T.L.J., and A.W.S. performed research; R.L.B., B.F.C., T.L.J., and A.W.S. analyzed data; andM.W.A., R.L.B., B.F.C., and T.L.J. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission. R.L.K. is a Guest Editor invited by the EditorialBoard.1To whom correspondence should be addressed. Email: mwallen@cpp.edu.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1607996113/-/DCSupplemental.

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threat of punishment and sanction enforced by powerful leaders,then (P2) we predict that rates of lethal aggression should covarypositively with sociopolitical complexity, increasing as complexityincreases.Here we evaluate the relative importance of resource scarcity

and sociopolitical complexity on rates of violence using an excep-tionally robust archaeological database of human burials that in-cludes remains from thousands of individuals who lived in centralCalifornia between 1,530 and 230 cal BP (31). Specifically, weevaluate how two forms of violence, sharp force and blunt forcecraniofacial trauma, vary relative to resource scarcity and politicalcomplexity (Fig. 1 and Table S1). Because we cannot distinguishbetween offensive vs. defensive violence, interpersonal vs. coali-tional lethal aggression, or intra- vs. intergroup violence from thearchaeological record, we treat these data as a long-term record ofoverall violence occurring in a given area. First, we summarize theevidence of violence occurring within the boundaries of each eth-nolinguistic group in central California. Then, using environmentalproductivity as a proxy for the relative utility of the local environ-ment (32) and ethnographic estimates of political complexity (33),we link the data on violence to these ecological and ethnographic

proxies to determine if rates of violence are driven more by re-source scarcity or political complexity.

ResultsAs shown in Table 1 and Fig. 2A, the proportion of individualssuffering from sharp force trauma significantly declines withenvironmental productivity, confirming the first prediction (P1)that resource scarcity increases lethal aggression. Sharp forcetrauma also varies significantly with political complexity (Table1), but contrary to the second prediction (P2), sharp forcetrauma is highest at intermediate levels of political leadership(Fig. 2B) and the extremes of political organization (Fig. 2C).Neither environmental productivity nor political organization

predicts the proportion of cases exhibiting blunt force traumaacross the study area, suggesting that different causal factors maybe driving this type of violence (Table 1). This interpretation issupported by significant variation between the frequency of bluntforce vs. sharp force trauma that resulted in or was at least as-sociated with mortality; a relatively low number of blunt forcetrauma cases were identified as definitively perimortem (23.53%;36/153), with the remainder being antemortem or indeterminate,whereas 94.39% (286/303) of the sharp force trauma was

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Fig. 1. Spatial distribution of (A) archaeological sites in the CCBD relative to contact-era ethnolinguistic boundaries, (B) proportion of sharp and blunt forcetrauma, (C) environmental productivity (NPP), and (D) political leadership and organization summarized for each ethnolinguistic group.

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recorded as definitively perimortem, likely causing or contrib-uting to the death of the individual.Controlling for the potential covariance between environ-

mental productivity and political complexity, which could con-flate the interpreted cause of these relationships, shows that theproportion of sharp force trauma varies significantly with envi-ronmental productivity alone (Table 1 and Fig. 3). This findingprovides support for the first prediction (P1) that violence isdriven by resource scarcity and suggests that political complexityhas little influence on violence independent of environmentalproductivity (refuting P2). As with the bivariate models, inmultivariate models, the proportion of individuals who experi-enced blunt force trauma does not vary significantly with any ofthe independent variables (Table 1).

DiscussionComparison of environmental productivity and sociopoliticalorganization relative to the skeletal record of violence over thelast 1,500 y of prehistory in central California shows only onestatistically meaningful correlation: negative covariance betweensharp force or projectile trauma (the most pervasive form ofviolence in the record) and net primary productivity (NPP).When accounting for environmental variation, our findingsprovide no indication that societies with more complex societalforms were more prone to intergroup or interpersonal violence.Although the range of variation in sociopolitical complexity islimited in this region relative to global variation, our Californiasample shows significant levels of, and variation in, violence andfurther demonstrates that violence has little or nothing to dowith sociopolitical complexity but rather with environmentalproductivity. Sociopolitical complexity may be a sufficient, but isnot a necessary cause of hunter-gatherer violence. Blunt forcecranial trauma does not correlate with either environmentalproductivity or political organization in the central Californiaprehistoric record. Given that the majority of incidents of bluntforce trauma are not associated with lethal violence, thesefindings suggest that it represents a different form of close-range,interpersonal conflict occurring in different environmental andpolitical contexts than projectile injuries.In this hunter-gatherer case, environmental productivity is a

stronger predictor of heightened levels of lethal aggression thanrelative sociopolitical complexity, supporting the notion that incontexts of resource scarcity, the perceived benefits for individualsto engage in lethal aggression may have outweighed the perceivedcosts. There are at least two ways to interpret this finding.On the one hand, low environmental productivity could be asso-

ciated with violence simply as a result of individuals experiencingmore frequent resource shortfalls. When such events occur, andpossibly even when they are anticipated, individuals may find itworthwhile to take resources or territories from their neighbors. Ifsuch events are frequent enough, this could lead to increased levelsof violence in low productivity regions. Individuals in low productivityenvironments may also be less tolerant of theft from neighbors.Given that the utility of a resource diminishes with the amount ofthat resource an individual possesses (23), those with more may bemore tolerant of theft from others, whereas those with less should beless tolerant of theft. As such, resource claims and competitions inlow productivity environments should more frequently result in epi-sodes of violence from those intolerant of stealing. Combined,these scenarios suggest that individuals in lower productivity envi-ronments may experience a higher risk of shortfall due to the loweramounts of food available within their foraging radius; therefore,they are more likely to travel into neighboring territories in searchof resources, where they encounter neighbors who are equally atrisk for shortfall and intolerant of theft.On the other hand, there are also reasons to suspect that the

inverse relationship between violence and environmental pro-ductivity we report might not be due to simple resource scarcity per

se, i.e., scarcity actually experienced by individuals, as intuitionmight suggest. If prehistoric populations distributed themselves tomaximize their rate of resource acquisition, which the evidencesuggests (32), then all individuals should have the same rate ofenergy gain regardless of environmental productivity, implying thatindividuals in resource-rich and resource-poor environments maybe equally likely to experience per capita resource scarcity. If this istrue, then the differences in violence between resource poor vs.rich environments may not result from differences in resource

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Fig. 2. Response plots illustrating significant bivariate model results: the effectof (A) environmental productivity, (B) political leadership, and (C) political or-ganization on the proportion of burials exhibiting sharp force trauma.

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scarcity, but from how those resources are distributed resulting inchanges in mobility and territory size. Populations in lower pro-ductivity environments have significantly larger territories (32) andgreater mobility within those territories (34). Because individuals inlow productivity environments must travel widely to obtain enoughresources, individuals in these environments may operate in poorlydefined territorial boundaries and may have less information abouttheir neighbor’s willingness to punish poachers, both of which maycause individuals from neighboring groups to come into conflict. Inthis scenario, violence varies inversely with resource productivity asthe result of disputes resulting from either conflicting territorialclaims or misunderstandings and misinformation, where lowpopulation densities translate into widely separated groups un-familiar with their neighbors and territorial boundaries. In these

circumstances, individuals are making optimal assessments abouthow to acquire resources across a large and unproductive land-scape on the basis of what little information they have regardingtheir neighbors, but this has the unintended consequence ofincreased violence.Regardless of which scenario underlies the negative correla-

tion between projectile violence and environmental productivity,these results add to a growing body of evidence suggesting thatrates of violence across small scale societies are driven by indi-vidual evaluations of costs and benefits. Rather than arguingwhether or not violence is an ancestral or derived characteristicof human societies, we suggest that future work should continueto examine variation in the rates of violence across populations

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Fig. 3. Response plots illustrating results of twomultivariate models. The first (A) examines variation in the proportion of burials exhibiting sharp force trauma as afunction of the combined effect of environmental productivity and political leadership. The second (B) examines the proportion of burials exhibiting sharp forcetrauma as a function of environmental productivity and political organization. When combined, only environmental productivity remains significant (Table 1).

Table 1. Summary of generalized additive model results examining the effect of EP, PL, and POon the proportion of burials with evidence for SFT and BFT within each ethnolinguistic group

Model type Prediction Dependent IndependentEstimated degrees

of freedom Proportion explained P

Bivariate P1 SFT EP 1.02 30.30 0.0008P2a SFT PL 1.83 31.00 0.0260P2b SFT PO 1.81 27.50 0.0078P1 BFT EP 1.31 7.81 0.4078P2a BFT PL 1.00 2.00 0.8016P2a BFT PO 1.80 22.10 0.1533

Multivariate P1+2a SFT EP 1.00 30.70 0.0091PL 1.52 0.3116

P1+2b SFT EP 1.00 37.80 0.0024PO 1.00 0.5892

P1+2a BFT EP 1.23 9.88 0.2716PL 1.00 0.2871

P1+2b BFT EP 1.23 37.30 0.2716PO 1.00 0.2871

Table shows the estimated degrees of freedom, the proportion explained, and the P value for each dependentand independent variable pair. Significant terms are highlighted in bold. BFT, blunt force trauma; EP, environ-mental productivity; PL, political leadership; PO, political organization; SFT, sharp force trauma.

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relative to indicators of resource scarcity to explain the un-derlying causes of violence throughout human history.

MethodsData Collection.Central California Bioarchaeological Database. The Central California Bio-archaeological Database (CCBD) was assembled by one of us (A.W.S.) over thelast two decades with information gathered from 329 archaeological sites;80% of them were excavated after 1975 because of threatened impacts frommodern development (31, 35). It includes information on a total of 16,820individual burials that date back as far as 5,000 cal BP from 19 ethnohistoricterritorial delineations (31, 35). All of these groups were relatively broad-spectrum hunter-gatherers, organized into a large number of autonomouspolities that are often aggregated by anthropological researchers into largergroups based on language. Three forms of violence well attested in theethnographic record are evident in this sample: blunt force cranial trauma,sharp force trauma, and trophy taking behavior. Across the entire database,the most pervasive form of violence is sharp force or projectile trauma,found in 7.4% of 6,278 assessed burials. It was significantly more commonamong males (10.7%) than females (4.5%) and most common among youngadult males. Indicated by cut marks, indentations, perforations on bones,and embedded projectile points, this form of trauma increased markedly infrequency during late prehistory, likely due to the introduction of the bowand arrow (31). Blunt force cranial trauma is the second most common formof injury, remaining relatively constant at ∼5% for most of prehistory foradult males and slightly less for adult females, but increasing after 500 calBP. Trophy-taking behavior is the practice of dismembering and displayingbody parts and was the least common form of violence in the CCBD, peaking2,500–1,500 y ago, with 4.2% of males and 1% of females being subjected toremoval of crania or postcranial elements. Recent research suggests thattrophy-taking may represent profoundly different underlying social andpolitical phenomena than projectile violence and blunt force trauma (36), soit is not further considered here.

For the current undertaking, we restricted our sample to burials repre-senting only the last 1,500 y of prehistory, which are most relevant to theethnographic record; this subsample includes 3,939 burials assessed for sharpforce trauma and 3,947 burials assessed for blunt force cranial trauma from127 sites (Table S1 and Fig. 1).Relative sociopolitical complexity. Relative complexity of California Nativehunter-gatherer societies (37, 38) was assessed for the 19 ethnolinguisticgroups represented in the CCBD with reference to two variables in Jorgenson’s(33) Western North American Indian database. These variables include (i) typeand complexity of political leadership (variable VII-A-332) and (ii) governmentand territory (variable VII-B-334). Although these values are based on obser-vations and accounts of ethnographic societies, these scores should still berepresentative of the general conditions experienced by individuals recorded inthe CCBD given that the archaeological (39) and linguistic (40) records suggeststrongly that ethnographic patterns likely emerged 1,500 y ago and that groupsmigrating into California had arrived in their historically observed locations bythat time or only slightly thereafter.

Variation in political leadership was mainly between groups with a singleleader (or headman) advised by an informal council of elders and groups witha single leader with one or more assistants and/or a formal council. Jorgenson’svariable VII-A-332 is formally titled “Type and Complexity of Political Leadershipin the Focal Local Community” and has 10 possible ordinal estimates for each ofthe ethnographic groups, but the ethnographic groups here represent only 3 ofthese: a score of 2 represents a single leader with at most a council of elders asadditional political offices; a score of 3 represents a single leader or headmanwith one or more functional assistants and/or a formal council or assembly, butwithout an elaborate or hierarchical organization. The majority of the ethno-linguistic groups are split fairly evenly between scores of 2 and 3. A score of 6represents “theocratic, authority being vested not in secular officials, but in apriesthood, a secret society, or other religious functionaries” (33, p. 610). Thislatter form of organization was found only among the Konkow Maidu.

Variation in political organization was mainly between social formationsconsisting of just one kin group (e.g., patrilineal bands) and formations consistingof multiple kin groups; the units are known in California as tribelets. Jorgensonvariable VII-B-334 is formally titled, “Government and Territory,” with up to 13possible ordinal scores. The ethnolinguistic sample used here again only repre-sents three of these possibilities. A score of 1 indicates a local society that has noterritorial organization larger than the residential kin group. True political or-ganization is lacking; a 2 is assignedwhere succession of the office of headman isthrough appointment by a higher political authority. This score is the vast ma-jority of the cases in our sample. A score of 3 is assigned where the local society iscomposed of several residential kin groups that are formally united into villagesor bands, and these political units are in turn combined with others to form atribe or district (33, p. 611). In our sample, this is found only among the Nisenan,and even there it is on the basis of somewhat circumstantial evidence (41).Environmental productivity. Environmental productivity values were taken fromCodding and Jones (32). In some cases, data on lethal aggression are assignedmore fine-grained territories than were available for the NPP data; in such cases,average NPP values are repeated for each ethnographic group. Mean NPP wascalculated for each ethnolinguistic group from a global raster of remotely senseddata from the MODIS instrumentation on NASA’s Terra satellite, processed andprovided by the Numerical Terradynamics Simulation Group at the University ofMontana (42, 43). NPP is an approximation of photosynthesis, measuring theamount of energy that is turned into mass and thereby approximating theamount of new growth biomass available to consumers. Although a crudemeasure of environmental variation, it does predict variation in hunter-gathererdemography and settlement patterns (32), suggesting that it is a reliable proxyof habitat quality and resource abundance. Additionally, although modern dataare used here to represent the last 1,500 y, the use of modern NPP is an ap-propriate proxy for past resource abundance given the scale of our analysis anddominance of a single, specifically Mediterranean (dry summer, wet winter) cli-matic pattern during the period of interest; this is in contrast to other areas(e.g., the southern Great Basin) whose climatic history shows major shifts be-tween quite different (e.g., dry summer Mediterranean vs. wet summer mon-soonal) climatic regimes, therefore preventing simple extrapolation from presentto past resource abundance. Mean NPP for our central California study area hascertainly varied, but the relative ranking of each ethnographic group should haveremained the same. Table S1 also reports data on territory size and populationdensity from Codding and Jones (32), with updated territory size estimates forsubdivided Miwok and Patwin linguistic regions following Kroeber (44, 45).

Analytical Methods. To determine whether each of the independent variables(environmental productivity, political complexity, and territorial organiza-tion) predicts variation in the dependent variables (the proportion of burialsexhibiting sharp or blunt force trauma), we rely on generalized additivemodels (GAMs) (46–48). Because these relationships may be nonlinear, GAMsallow for the underlying trends within the data to emerge without anymajor assumptions by the investigator. All models use a binomial distribu-tion and log link appropriate to proportional data and follow a quasi-like-lihood estimation to reduce the chances of overdispersion. To maximizeparsimony, we minimize the degrees of freedom (or knots) to the minimumpossible (k = 3). In addition to bivariate models, we also construct multi-variate models to control for the interaction between each of the in-dependent variables. To account for variation in sample size, all modelsweight each data point by the total number of observations (burials) fromwhich the proportion is calculated. Model results report the estimated de-grees of freedom of the smooth term, the proportion of deviance explainedby the inclusion of the independent variable (also known as the likelihood r2, orR2L), and the α or P value associated with each independent variable.

ACKNOWLEDGMENTS. The authors thank the anonymous reviewers, EarleKeefe, Shane Macfarlan, Robert L. Kelly, and Richard G. Klein for insightfulcomments on the manuscript.

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